Diesel engines depend on the heating of a combustible mixture of fuel and air by compression. During starting of a diesel engine when the engine is cold, the compressive heating may not be sufficient to produce ignition. It has therefore been the practice to provide diesel engines with glow plugs to provide auxiliary heating during starting to produce ignition of the air-fuel mixture. The lower the ambient temperature conditions during the starting operation, the greater the amount of heat that must be supplied by the glow plugs to initiate combustion. Heretofore it has been the practice to preheat the glow plugs by connecting the glow plugs across the battery of the vehicle for sufficient time to allow the glow plugs to come up to the necessary operating temperature before engaging the starter. When the engine is started at relatively low ambient temperatures, such as may be experienced in the winter time in frigid areas, the heating time may be annoyingly long. While the heating time can be reduced by increasing the current to the glow plugs, this may result in damage to the glow plugs due to temperature overshoot. While automatic control circuits for diesel glow plugs have heretofore been proposed, such known circuits have merely controlled the timing of the turning on and turning off of the glow plugs, but have not controlled the temperature level of the glow plugs as a function of ambient temperature conditions.